1. Field of the Invention
The present invention relates in general to a hydraulic control apparatus for a vehicular fluid-actuated power transmitting device provided with a lock-up clutch, and more particularly to improvements of a cooler by-pass valve used in the hydraulic control apparatus.
2. Discussion of the Related Art
In the field of an automotive vehicle including a fluid-actuated power transmitting device such as a torque converter or fluid coupling provided with a lock-up clutch, there is known a hydraulic control apparatus arranged to place the lock-up clutch in a slipping state for absorbing a periodic torque variation of an engine of the vehicle during running of the vehicle at a relatively low speed. A hydraulic control apparatus 200 shown in FIG. 5 is an example of such a known hydraulic control apparatus. This hydraulic control apparatus 200 includes: a solenoid-operated switching valve 202 operable to place a lock-up clutch 201 in a selected one of an engaged state and a released state; a lock-up relay valve 204 operable between two positions thereof in response to an operation of the solenoid-operated switching valve 202; a lock-up control valve 206 operable to control the engaged state of the lock-up clutch 201 by regulating the pressure of a working oil or fluid to be discharged from a fluid-actuated power transmitting device in the form of a torque converter 210, when the lock-up relay valve 204 is placed in the position for placing the lock-up clutch 201 in the engaged state; and a linear solenoid valve 208 operable to generate a pilot pressure which acts on a spool of the lock-up control valve 206, for controlling a pressure regulating operation of the lock-up control valve 206. The lock-up control valve 206 controls the engaged and released states of the lock-up clutch 201, and the slipping state which is a transient state between the engaged and released states.
The torque converter 210 is provided with a pump impeller 212 and a turbine impeller 214. While the lock-up clutch 201 is placed in the released state, the working fluid in the torque converter 210 is stirred or agitated by the pump and turbine impellers 212, 214, so that the temperature of the working fluid rises during an operation of the torque converter 210. To cool the heated working fluid, the hydraulic control apparatus 200 is provided with an oil cooler 216. The hydraulic control apparatus 200 is further provided with a cooler by-pass valve 218 for preventing a damage or any other trouble with the oil cooler 216 due to a rise of the pressure of the working fluid in an oil passage (e.g., cooler hose), which takes place due to partial plugging of the oil passage with impurities contained in the working fluid. When the pressure of the working fluid in the oil cooler 216 rises above a predetermined upper limit, the cooler by-pass valve 218 is opened to prevent a further flow of the working fluid into the oil cooler 216. JP-10-267115 A discloses a hydraulic control apparatus in which the cooler by-pass valve has different functions at different levels of the pressure of the working fluid, so that the number of the required components of the hydraulic control apparatus is reduced, whereby the cost of manufacture of the apparatus is accordingly reduced.
Usually, the working fluid has properties that permit the vehicle to be driven with a reduced amount of consumption of a fuel by the engine, when the temperature of the working fluid is held within a preferred range of about 80-90° C. In this respect, it is desirable to rapidly raise the temperature of the working fluid to a level within the preferred range, immediately after starting of the vehicle, for improving the fuel economy of the vehicle. However, the known hydraulic control apparatus as disclosed in the above-identified publication is arranged such that the cooler by-pass valve is controlled solely on the basis of the pressure of the working fluid, so that the working fluid is cooled by the oil cooler immediately after starting of the vehicle, namely, even while the temperature of the working fluid is still lower than the lower limit of the preferred range. Accordingly, the rise of the working fluid temperature is delayed after starting of the vehicle, unfavorably resulting in a failure to improve the fuel economy of the vehicle.